Volcanic Ash Cloud w/ Umbrella

[ecarlsen]

Hi all,

I've used Maya professionally for several years but only just started with Phoenix FD for Maya, and am seeking suggestions on how to approach a fluid sim.

I am trying to create a specific volcanic eruption with a large pronounced umbrella shape at the top, similar to this image: https://en.wikipedia.org/wiki/Erupti...doubtedit1.jpg. This other image shows a bit of how an how the umbrella shape naturally starts to form: https://media.wired.com/photos/5926c...aguito_ann.jpg

My first guess was to start with the preset heavy smoke and set up a torus-shaped volume axis field higher up to make the big umbrella, though it has the potential to create a bit of a weird boomerang look, with the smoke lifting above it then getting kind of pulled back down into the shape. I also considered creating a second sim for the umbrella shape that starts to emit when the main ash plume has reached that height.

Any tips on how you would approach this? Maybe there's an elegant way to do it purely by tweaking a single sim's settings, so it has enough upward thrust to stay straight, but then naturally spreads out into a wide umbrella at the top?

Thanks in advance for any ideas!

-Eric

[Raven]

To me, that is the smoke reacting to different thermal layers in the atmosphere.
I think you could try using body force objects with low max distance so they only affect the smoke when it reaches them to spread the smoke at different layers.

[ecarlsen]

Thanks so much for your response, Gavin. I didn't know about Body Force objects and they seem like the right direction.

Unfortunately I haven't gotten it to work just yet. I've tried several tests using different objects as the Body Force object, and within the Extra Phoenix FD Attributes of the the object's shape node, I unchecked Solid, to prevent it from acting as a collision object. However, when the smoke reaches any of these objects, it gets pulled rapidly into the center of the volume, no matter the size/shape. I've attached a screenshot (the object in this one is a wide flattened sphere).

Do you know how I could have it spread out to fill the object's shape?

[Raven]

I have not had a chance to try is with smoke, but try with little or no strength and use the spread speed.

From the docs.
Spread Speed | insidespreading ? Specifies the rate at which the fluid fills an object's volume evenly in world units per second.

[Raven]

Having less strength did not seem to work, here is what I got doing a quick test in Max.
With my settings attached

[ecarlsen]

Hi Gavin, thanks so much for your reply. I've tried your settings and done a bunch of tinkering with the parameter numbers, but still seem to get the same results every time, where it pulls all the smoke into a tight area in the center. I've attempted with a squashed sphere and with a cylinder for Body Force.

It seems to always be pulling it into a tight 2 unit-wide area, as if it thinks the object is much smaller than it really is, but when I set the object to Solid, the smoke properly collides with the full-size shape. So I'm not sure it's a scene scale problem.

I wonder if this problem could be related to the simulation settings? I'm starting from the "Heavy Smoke" preset and lowering the grid resolution a bit. Are you using custom settings different than this?

[Svetlin.Nikolov]

Hmmm, note that copying the same numbers from Max would only work if you have set Maya to work in millimeters. Otherwise, e.g. in cm, you need to set Max distance to 100 and Damp min vel. to 5.8.

Cheers!

[Raven]

I did it with default settings, but busy running a sim now with the large-scale smoke preset and it seems to be fine as well.

As Svetlin says, I didn't pay much attention to my units, so you may need to upscale them quite a bit.

[Raven]

Large-scale test.
Eric, what are your units and size of your source object, I'll see if I can replicate in the same units as your scene.

[ecarlsen]

Ah what you came up with looks so good! I wish mine was behaving the same I tried adjusting the numbers as Svetlin suggested because I'm in centimeters, with Max distance to 100 and Damp min vel. to 5.8, but still get the same kind of result.

I'm emitting from a sphere, 20 subdivisions on axis & height, radius of 1 (cm). Body Force object is a squashed sphere (also 20 axis/height subdivisions); X/Y dimensions are 13.192cm and height is 1.749cm.

[ecarlsen]

Another weird thing - I tried setting my scene to millimeters, but now when I apply PhoenixFD to an object, it emits from a much smaller version of it in the middle, and behaves strangely. I've attached an example emitting from a plane, but it does the same for a sphere. Note this is at frame 3.

Is there some other setting that needs to be changed apart from changing Maya's scene units? Maybe this is all tied into the issue with the Body force object?

[ecarlsen]

Guys, I think it's working! The key was indeed to divide things by 10 as you'd suggested, but I had only divided Max Distance and Damp Min Vel by 10. Once I divided the Spread Speed by at least 10 as well, the behavior became much more natural, and now gently fills in the general shape of the Body Force object, rather than getting sucked into a small area in the middle.

I've still got a lot of tweaking to do to get the look I want, but I'm super happy to have a system in place now. Thanks SO much for your help on this!

[Svetlin.Nikolov]

Ahhhh, you're absolutely right, Spread Speed needs to be divided as well... Looking good!

[Raven]

Glad we could help.
Svetlin.Nikolov, I do think a "thermal body" would also be an awesome feature. So you could create an object and then control the temperature just inside that object to either heat or cool systems.

[LarsSonparsson]

Can't you do it with another source, that doesn't have any other values active than temperature? So you could have an object filled with cool air, but it wouldn't affect velocity or other channels